The present invention relates to enamellings with a heterogeneous structure having at least two different phases. These enamellings are distinguished by their high mechanical and thermal stability, low thermal coefficient of expansion, high insulation resistance and excellent electrical properties.
Metal cores covered with such enamels are particularly suitable for use as substrates for the assembly of printed circuit boards and electronic circuits. The layer of enamel acts as insulator on which metallic circuit structures may be applied. With the addition of active (IC) and passive components, such printed circuit boards may be used for the construction of electronic component groups.
For many years it has been known to those skilled in the art of electronic circuits that electronic circuit boards could be constructed from substrates such as phenol resin impregnated paper and epoxy glass as well as Al.sub.2 O.sub.3. Each of those substrates has their own drawbacks for use as electronic circuit board substrates.
The conventional synthetic resin circuit boards, for example, no longer satisfy the physical and electric requirements arising in many cases from new semiconductor components. On account of the high stray powers of some electronic components, auxiliary constructions are required for the removal of heat.
Al.sub.2 O.sub.3 substrates which are used in hybrid technology show an improvement in physical and electrical properties in this respect. However, the range of application of Al.sub.2 O.sub.3 ceramics is limited by brittleness to substrate sizes of less than 10.times.10 cm.
With a view to overcoming the abovementioned difficulties of these technologies, metal substrates coated with enamel have in recent times been used as bases for printed circuit boards. It was found that the advantageous physical and electrical properties could then be combined with excellent mechanical stability.
Enamel, like all other vitreous materials, has a wide softening range which is dependent upon the temperature. The result is that, in the known enamelling technology, stoving of the enamel layer must lower the viscosity sufficiently to enable the enamel particles to fuse together, but sufficient hardness or strength can only be obtained by cooling to a considerably lower temperature. The difference between the stoving temperature and the maximum use temperature is normally from 300.degree. to 400.degree. C.
Thus if the screen printing technique were used, for example, for the manufacture of printed circuits on enamelled steel substrates and the enamel layer were stoved at a temperature of 850.degree. C., then the conductive tracks would have to be stoved at a temperature of not over 550.degree. C.
Although special types of enamel could be developed, even those would soften at temperatures higher than about 620.degree. to 650.degree. C. so that it would be necessary to develop suitable screen printing pastes for such maximum temperatures. It would be desirable to use the usual pastes employed for ceramic substrates but a simple shift in the softening range to higher temperatures is contraindicated by the fact that structural changes take place in steel at temperatures of 900.degree. C. and higher, and these changes inevitably result in a certain permanent deformation of the substrate which can then no longer be kept flat as required.